CVE-2023-53809: 
Linux Kernel vulnerability analysis and mitigation

In the Linux kernel, the following vulnerability has been resolved:

l2tp: Avoid possible recursive deadlock in l2tp_tunnel_register()

When a file descriptor of pppol2tp socket is passed as file descriptor of UDP socket, a recursive deadlock occurs in l2tp_tunnel_register(). This situation is reproduced by the following program:

int main(void) { int sock; struct sockaddr_pppol2tp addr;

sock = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
if (sock < 0) {
	perror("socket");
	return 1;
}

addr.sa_family = AF_PPPOX;
addr.sa_protocol = PX_PROTO_OL2TP;
addr.pppol2tp.pid = 0;
addr.pppol2tp.fd = sock;
addr.pppol2tp.addr.sin_family = PF_INET;
addr.pppol2tp.addr.sin_port = htons(0);
addr.pppol2tp.addr.sin_addr.s_addr = inet_addr("192.168.0.1");
addr.pppol2tp.s_tunnel = 1;
addr.pppol2tp.s_session = 0;
addr.pppol2tp.d_tunnel = 0;
addr.pppol2tp.d_session = 0;

if (connect(sock, (const struct sockaddr *)&addr, sizeof(addr)) < 0) {
	perror("connect");
	return 1;
}

return 0;

}

This program causes the following lockdep warning:

============================================ WARNING: possible recursive locking detected 6.2.0-rc5-00205-gc96618275234 #56 Not tainted

repro/8607 is trying to acquire lock: ffff8880213c8130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: l2tp_tunnel_register+0x2b7/0x11c0

but task is already holding lock: ffff8880213c8130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppol2tp_connect+0xa82/0x1a30

other info that might help us debug this: Possible unsafe locking scenario:

    CPU0
    ----

lock(sk_lock-AF_PPPOX); lock(sk_lock-AF_PPPOX);

*** DEADLOCK ***

May be due to missing lock nesting notation

1 lock held by repro/8607: #0: ffff8880213c8130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppol2tp_connect+0xa82/0x1a30

stack backtrace: CPU: 0 PID: 8607 Comm: repro Not tainted 6.2.0-rc5-00205-gc96618275234 #56 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014 Call Trace: dump_stack_lvl+0x100/0x178 __lock_acquire.cold+0x119/0x3b9 ? lockdep_hardirqs_on_prepare+0x410/0x410 lock_acquire+0x1e0/0x610 ? l2tp_tunnel_register+0x2b7/0x11c0 ? lock_downgrade+0x710/0x710 ? __fget_files+0x283/0x3e0 lock_sock_nested+0x3a/0xf0 ? l2tp_tunnel_register+0x2b7/0x11c0 l2tp_tunnel_register+0x2b7/0x11c0 ? sprintf+0xc4/0x100 ? l2tp_tunnel_del_work+0x6b0/0x6b0 ? debug_object_deactivate+0x320/0x320 ? lockdep_init_map_type+0x16d/0x7a0 ? lockdep_init_map_type+0x16d/0x7a0 ? l2tp_tunnel_create+0x2bf/0x4b0 ? l2tp_tunnel_create+0x3c6/0x4b0 pppol2tp_connect+0x14e1/0x1a30 ? pppol2tp_put_sk+0xd0/0xd0 ? aa_sk_perm+0x2b7/0xa80 ? aa_af_perm+0x260/0x260 ? bpf_lsm_socket_connect+0x9/0x10 ? pppol2tp_put_sk+0xd0/0xd0 __sys_connect_file+0x14f/0x190 __sys_connect+0x133/0x160 ? __sys_connect_file+0x190/0x190 ? lockdep_hardirqs_on+0x7d/0x100 ? ktime_get_coarse_real_ts64+0x1b7/0x200 ? ktime_get_coarse_real_ts64+0x147/0x200 ? __audit_syscall_entry+0x396/0x500 __x64_sys_connect+0x72/0xb0 do_syscall_64+0x38/0xb0 entry_SYSCALL_64_after_hwframe+0x63/0xcd

This patch fixes the issue by getting/creating the tunnel before locking the pppol2tp socket.